Metathesis of olefins is a process that is defined as the redistribution of alkylidene moieties in a mixture of olefins to yield other olefins. A simple example of olefin metathesis is shown in equation I. EQU 2RHC.dbd.CHR'.dbd.R'HC.dbd.CHR'+RHC.dbd.CHR Eq. I
The reaction proceeds by addition of an olefin to a catalyst having a metal-carbon double bond. Three of the most active metals used in classical olefin metathesis are molybdenum, tungsten and rhenium. (Ivin, K. J., Olefin Metathesis, Academic Press, London, 1983; Grubbs, R. H. in Comprehensive Organometallic Chemistry, Wilkinson, G. et al. (Eds.), Vol. 8, Pergamon: New York (1982); Dragutan, V. et al., Olefin Metathesis and Ring-Opening Polymerization of Cyclo-Olefins, 2nd Ed., Wiley-Interscience: New York (1985); Leconte, M. et al. in Reactions of Coordinated Ligands, Braterman, P. R. (Ed.), Plenum: New York (1986).)
Examples of molybdenum (VI) alkylidene complexes (Murdzek, J. S. and R. R. Schrock, Organometallics 6:1373 (1987); Bazan, G. et al., Polymer Comm. 30:258 (1989)) and tungsten (VI) alkylidene complexes have been previously described (Schrock, R. R. et al., J. Am. Chem. Soc. 110:1423 (1988); Feldman, J. et al. in Advances in Metal Carbene Chemistry, Schubert, U. (Ed.). Kluwer Academic Publishers, Boston: 1989. page 323; Schrock, R. R. et al., Macromolecules 20:1169 (1987); Ginsburg, E. J. et al., J. Am. Chem. Soc. 111:7621 (1989); Swager, T. M. et al., J. Am. Chem. Soc. 111:4413 (1989); Knoll, K. and R. R. Schrock, J. Am. Chem. Soc. 111:7989 (1989); Schlund, R. et al., J. Am. Chem. Soc. 111:8004 (1989)). Several of these compounds have been shown to catalyze the metathesis of olefins with an activity that can be controlled through the choice of the alkoxide ligand. For example. tungsten and molybdenum catalysts reported by Schrock. R. R (U.S. Pat. Nos. 4,681.956 and 4.727.215) have been shown to homogeneously metathesize at least 250 equivalents of methyl oleate. Though the reported molybdenum and tungsten catalysts can metathesize ordinary olefins (hydrocarbon chains) in good yield, they are limited in their usefulness as metathesis catalyst for functionalized olefins due to their reactivity with the functional groups.
Several rhenium alkylidene complexes have also been reported (Edwards. D. S. et al., Organometallics 2:1505 (1983); Edwards. D. S., "Synthesis and Reactivity of Rhenium (VII) Neopentylidene and Neopentylidyne Complexes". MIT Doctoral Thesis (1983); Horton, A. D. et al., Organometallics 6:893 (1987); Horton, A. D. and R. R. Schrock, Polyhedron 7:1841 (1988); Cai, S. et al., J. Am. Chem. Comm., 1489 (1988). In particular, the Edwards references describe three rhenium complexes represented by the formula Re(C-t-Bu)(CH-t-Bu)(R).sub.2 where R is a t-butoxide, trimethylsiloxide or neopentyl moiety. However, none of the previously reported rhenium compounds showed any confirmable metathesis activity in the absence of a co-catalyst or activator compound, even toward strained cyclic olefins, such as norbornene.
Heterogeneous rhenium catalysts (Re.sub.2 O.sub.7 deposited on silica and/or alumina mixtures) have been shown to metathesize methyl oleate but for a limited duration before becoming inactive.
It would, therefore, be desirable to provide a homogeneous rhenium catalyst for metathesizing olefins, particularly functionalized olefins, at a molecular level which would be highly active, longer-lived than heterogeneous rhenium catalysts and tolerant of olefin functionalities.